Displays can be created from an array of organic light emitting diodes (“OLEDs”) each controlled by individual circuits (i.e., pixel circuits). The individual circuits have transistors for selectively controlling the circuits to be programmed with display information and to emit light according to the display information. OLEDs are emissive display devices which generally emit light according to the amount of current driven through the OLED. OLEDs generally include a light emitting region where positively charged holes meet with electrons. Light is emitted as the electrons are captured by the holes and settle at a lower energy state. The amount of current driven through the OLED is thus proportionate to the number of emission events, and the light emitted from an OLED is accordingly related to the current driven through the OLED. Thin film transistors (“TFTs”) fabricated on a substrate can be incorporated into such displays to control the amount of current driven through the OLEDs according to the display information programmed into the individual circuits.
OLEDs can be developed by sequentially depositing layers of material onto a substrate. Such a layering process generally commences and terminates with depositing conductive electrodes (i.e., terminals) such that a completed OLED includes a plurality of layers disposed between two electrodes. To connect the OLED to a TFT of a pixel circuit, an electrical connection is generally made between a terminal of the TFT and one of the electrodes of the OLED through a contact, which process leads to problems due to the precision of the required alignment between the contacts and the OLED terminal and the relative unreliability and inefficiency of the contacts formed.
Applying a voltage across the two electrodes in excess of an operating voltage associated with the OLED generally allows a current to flow through the device and for light to be emitted from an emission region of the OLED. As the OLED ages, the operating voltage of the OLED can shift (e.g., increase). The shift in the OLED operating voltage influences the voltage applied across the TFT, and thereby modifies the current flowing through the OLED, and thus influences the light output of the OLED.
It is desirable, therefore, to configure the pixel circuit such that the terminal of the TFT coupled to the OLED does not influence the voltage applied across the TFT. Such a structure is commonly referred to as a reverse OLED, because one way to develop the structure is to sequentially develop the layers of the OLED in the reverse order. One way to develop a reverse OLED is to start the deposition on the display substrate with the cathode terminal (“layer”) instead of the anode terminal (“layer”). However, suitable transparent materials for use as a cathode terminal with a suitably high work function are rare, unavailable and/or expensive. Furthermore, the performance of such devices as have been created is inferior to conventional OLED devices.
Another method for achieving the desired structure is to develop the normal OLED on encapsulation glass and develop a matching contact on the TFT substrate. The two substrates can then be put together. However, the contact quality between the OLED and the matching contact requires careful alignment and consistent pressure. The results across an entire display are not good and displays created with such techniques frequently contain many dead pixels and high voltage OLEDs due to the poor quality of the electrical path between the contact and the OLED.